1. The Field of the Invention
The present invention relates to a method for reducing NOx during combustion of coal, more particularly to a method that utilize an organically complex nanocatalyst applied to or mixed with coal in order to catalyze the release of nitrogen from the coal prior to combustion in a low oxygen zone of a coal burner.
2. Related Technology
Coal combustion is major source of energy for the production of electricity throughout the world. Coal is a good source of energy because of its high energy to weight ratio and its great abundance. The use of coal, however, is increasingly under scrutiny because of environmental concerns. Among the known environmental difficulties with coal combustion is the production and emission of NOx compounds, such as NO, N2O, and NO2. NOx compounds can be very harmful to human health and are known to produce undesirable environmental effects such as smog.
Government regulations require emission from coal burning to be monitored and controlled. Controlling NOx emissions has become increasingly difficult as government regulations continue to lower the allowable level of NOx and other pollutants that can be released into the environment. The requirement for reduced pollutants from coal-fired power plants has led to a demand for suitable new technologies.
In a coal fired power plant, there are two principle sources of NOx formation: fuel NOx and thermal NOx. Fuel NOx is NOx that forms from nitrogen found in the fuel, whereas thermal NOx is formed from other sources of nitrogen such as nitrogen in the air. About 80% of NOx emissions from coal combustion are produced from fuel nitrogen.
One method used to reduce pollutants during coal combustion focuses on removing NOx from power plant flue gas. For example, NOx emitted in flue gas can be removed using selective catalytic reduction (SCR), which converts NOx compounds to nitrogen gas (N2) and water. However, this type of NOx control method is expensive, in part, because of the required capital investment. The cost of these technologies and increasingly stringent government regulations have created a need for less expensive technologies to reduce NOx emissions from coal combustion.
Another method of reducing NOx emissions is to remove coal nitrogen from the coal material by converting it to N2. Recently, researchers have discovered that iron-based catalysts can assist in releasing fuel nitrogen from coal. In work by Ohtsuka and coworkers at Tohoku University (Sendai, Japan), methods have been described for the production of an iron-based catalyst, which, when combined with coal and placed in an pyrolysis environment, causes nitrogen compounds in coal to be released more rapidly, thus causing a decrease in the amount of nitrogen remaining in the char material (Ohtsuka et al., Energy and Fuels 7 (1993) 1095 and Ohtsuka et al., Energy and Fuels 12 (1998) 1356).
Several features of the catalyst and methods used by Ohtsuka make such catalyst and methods too expensive and less effective than desired for use in coal fired power plants. First, Ohtsuka teaches precipitating a FeCl3 solution directly onto the coal using Ca(OH)2. Precipitating the catalyst onto the coal results in intimate contact between the coal and the catalyst precursors and other reagents used to make the catalyst nanoparticles. While Ohtsuka suggests washing the coal to remove chloride and calcium, this step requires washing the entire coal feedstream, which would be very costly on an industrial scale. Furthermore, at least some of these chemicals are likely to be adsorbed by the coal and remain even after washing. Introducing compounds such as chloride and calcium can have an adverse effect on power plant equipment and can cause pollution themselves.
In addition, precipitating the catalyst onto the coal requires that the catalyst be formed in the same location as the coal. This limitation could require that the catalyst be prepared at a coal mine or power plant, or that the coal material be shipped to a separate facility for catalyst preparation, thereby adding to production costs.
Another disadvantage of Ohtsuka's catalyst is that it requires high loading amounts to obtain desired results (e.g., up to 7% by weight of iron). High loading amounts can increase costs and offset the benefits of using a relatively inexpensive material such as iron. In addition, high iron content contributes to ash formation and/or can alter the ash composition.
Therefore, what is needed is an improved method for reducing NOx emissions during coal combustion, particularly a method that employs a catalyst that can more easily be manufactured, shipped, and/or stored, and then added to coal in lesser quantities than existing methods while still producing a desired reduction in NOx emissions.